Add missing virtual destructor to backend callbacks.

templates/TypeBuilder_h.template

 // This file is generated
 
 // Copyright (c) 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef {{"_".join(config.protocol.namespace)}}_{{domain.domain}}_h
 #define {{"_".join(config.protocol.namespace)}}_{{domain.domain}}_h
 
 {% if config.protocol.export_header %}
@@ skipping 186 matching lines @@
       {%- for parameter in command.returns -%}
         {%- if "optional" in parameter -%}
             Maybe<{{resolve_type(parameter).raw_type}}> {{parameter.name}}
         {%- else -%}
                 {{resolve_type(parameter).pass_type}} {{parameter.name}}
         {%- endif -%}
         {%- if not loop.last -%}, {% endif -%}
       {%- endfor -%}
         ) = 0;
         virtual void sendFailure(const DispatchResponse&) = 0;
+        virtual ~{{command.name | to_title_case}}Callback() { }
     };
     {% endif %}
     {%- if not("async" in command) %}
     virtual DispatchResponse {{command.name}}(
     {%- else %}
     virtual void {{command.name}}(
     {%- endif %}
     {%- for parameter in command.parameters -%}
       {%- if not loop.first -%}, {% endif -%}
       {%- if "optional" in parameter -%}
@@ skipping 70 matching lines @@
     static const char commandPrefix[];
     static const char version[];
 };
 
 } // namespace {{domain.domain}}
 {% for namespace in config.protocol.namespace %}
 } // namespace {{namespace}}
 {% endfor %}
 
 #endif // !defined({{"_".join(config.protocol.namespace)}}_{{domain.domain}}_h)